Color, digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED display device, each display element or pixel, is composed of red, green, and blue colored OLEDs. By combining the illumination from each of these three OLEDs in an additive color system, a wide variety of colors can be achieved.
OLEDs may be used to generate color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum. However, the known red and blue emissive materials are not particularly power efficient. In fact, broad bandwidth (appearing white) materials are known that have power efficiencies that are high enough by comparison to narrow bandwidth materials to produce a comparably power efficient OLED display by placing color filters over a broad bandwidth emissive material.
While power efficiency is always desirable, it is particularly desirable in portable applications because an inefficient display limits the time the device can be used before the power source is recharged. Portable applications may also require the display to be used in locations with high ambient illumination, requiring the display to provide imagery with a high luminance level to be useful, further increasing the power required to present adequate imagery.
In addition to the power efficiency of a display, another desirable attribute of a full color OLED display device is a reasonable color gamut. Color gamut is typically described by plotting the CIE chromaticity coordinates of the red 2, green 4 and blue 6 OLEDs in a CIE diagram as shown in FIG. 1. It should be appreciated, however, that to maximize the color gamut, the CIE chromaticity coordinates of the red 2, green 4, and blue 6 OLEDs must be separated as much as possible and placed on or very near the boundary of the CIE diagram. However, all of the colors on the boundary of this diagram are monochromatic and any source that can be placed near this boundary, by definition, emits light over a very narrow spectral band. For this reason, to create a display with a broad color gamut, the OLEDs must emit energy in a narrow spectral bandwidth.
Unfortunately, when a narrow bandwidth color filter is placed over a broad bandwidth light source to create an OLED element that emits light of a single color, the color filter absorbs more of the luminance energy of the illumination source than when a broad bandwidth filter is placed over the broad bandwidth light source. Similarly in OLED display devices that contain OLED materials that are doped to emit energy over narrow spectral bandwidths, the power efficiency of the OLED is reduced significantly.
The use of a broad bandwidth light emitting component is known in the prior art. For example, EP0830032B1 by Sampsell, Mar. 20, 2002, describes a projection system using a white light beam in conjunction with colored beams to improve the brightness of a projected display. Such a projection system, however, is not useful for an OLED display device. Further, this patent describes using the broad bandwidth white in a way that all colors will be perceived as lower in saturation. U.S. Pat. No. 5,526,016 issued Jun. 11, 1996 to Nakagiri et al., describes a rotary element of different colors to create a multi-color projection display. This projection device utilizes a white filter element to reduce the visibility of color breakup artifacts that are inherent in other similar color sequential display devices. This device is not suitable for small and compact display devices as required by portable application, has limited resolution, and is of very large size.
U.S. Pat. No. 5,638,084 issued Jun. 10, 1997 to Kalt, describes an electrostatically actuated display using red, green, blue, and white shutter elements. While the use of the white elements does improve the luminance of the display under certain circumstances, it does not alter the power used by the display system and the use of the white elements directly reduces that saturation of the displayed image to provide a higher luminance image.
U.S. Pat. No. 5,563,621 issued Oct. 8, 1996 to Silsby, describes a sixteen color display device that employs a pixel composed of red, green, blue, and white light emitting elements. While the addition of the white light emitting element allows the display to create four levels of gray through combinations of turning the white light emitting element on or off in combination with turning the red, green, and blue light emitting elements on and off, this patent does not discuss OLED display devices, nor does it discuss the creation of a full color display using a combination of red, green, and blue OLEDs.
U.S. Pat. No. 6,388,644 issued May 14, 2002 to DeZwart et al., describes a plasma or field emission display having “extra (non-saturating) phosphors” but does not address the need for reducing the power used by the display. As used in the '644 patent, the term “saturation” refers to an electronic saturation phenomenon relating to the efficiency of emission of photons from a phosphor that is stimulated by ultra violet photons, not color saturation as used in the present application.
There is a need, therefore, for an improved full color OLED display device having improved power efficiency while maintaining accurate color reproduction, including the saturation of in-gamut colors. As used below, the term “saturation” refers to color saturation (i.e. the purity of a color produced by the display device).